This invention relates to a disk array system. More particularly, the invention relates to a technology suitable for a disk array system which is used as a large capacity data storage apparatus and for which high reliability and high response to an input/output request are required.
An array system called “RAID (Redundant Array of Inexpensive Disks)” that is constituted by preparing a large number of hard disk drives as storage devices and causing them to distributedly store data has been put in the past into practical application.
This array system uses dozens to hundreds of storage devices (hard disk drives) and selects a plurality of storage devices among them (four drives in the case of 3D+1P in RAID number 5, for example) and a group of these storage devices constitute an RAID group. It will be assumed that eight SCSI (Small Computer System Interface) interfaces are connected to one channel control portion. It will be further assumed that a set of storage devices are connected to one interface, another set of storage devices are connected to another interface, and, similarly, other six sets of storage devices are connected to the remaining six interfaces, respectively. In the case of interface of the SCSI system, the maximum number of devices (storage devices) that can be connected to one interface is 16. Therefore, because eight interfaces are disposed in the example given above, maximum 8×16=128 storage devices can be connected.
The storage devices connected in this way are grouped into RAID groups each containing a predetermined number of storage devices. A parity bit is generated for a data block inputted to the apparatus and the data in the data block is divided and stored in the storage devices in the group. The parity is stored in another storage device in the group, too. Eight storage devices each connected to separate SCSI interfaces constitute one group. Seven storage devices in the group divide and store the data and the remaining one storage device stores the parity. For example, in a “RAID number 5” case, such a storage construction is called a “group of 7D+1P construction” (so-called “RAID group”). The number of the storage devices constituting the group varies depending on an application that is used, a data quantity handled, an object of use, and so forth.